Oil-injection screw compressor

Abstract

An oil-injection screw compressor includes a blow-off valve disposed on the discharge side of the compressor body and capable of releasing a quantity of air beyond the discharge capacity of the compressor body when operating of the motor at the minimum number of revolutions; a suction non-return valve which, disposed on the suction side of the compressor body, is closed when it is at a stop; and a pressure sensor for detecting the discharge side pressure of the compressor body; and, a controller which, at the time of operation of the motor at the minimum number of revolutions, opens the blow-off valve when the discharge side pressure P detected by the pressure sensor has risen to a prescribed upper limit Pu and closes the blow-off valve when it has fallen to a prescribed control pressure Po.

1. An oil-injection screw compressor in which oil is mixed into compressed gas, the oil-injection screw compressor comprising:
 a compressor body for compressing sucked air; 
 a motor for driving said compressor body; 
 an inverter for variably controlling a number of revolutions of said motor; 
 a blow-off valve disposed on a discharge side of said compressor body and capable of releasing a quantity of air beyond the discharge capacity of said compressor body when operating said motor at a minimum number of revolutions; 
 a suction non-return valve which, disposed on a suction side of said compressor body, is configured to be opened when said compressor body is operating under load and under no load, and closed when it is at a stop; and 
 a pressure detecting device for detecting a discharge side pressure of said compressor body, 
 wherein it is controlled that, at the time of operation of said motor at the minimum number of revolutions, said compressor is run under no load to open said blow-off valve in a condition of the operation of said motor at the minimum number of revolutions when the discharge side pressure of said compressor body detected by said pressure detecting device has risen to a prescribed upper limit, and said compressor is run under load by closing said blow-off valve when it has fallen to a prescribed control pressure during the no-load running, and 
 wherein it is controlled that, when said motor is stopped after the operation of said motor at the minimum number of revolutions, said blow-off valve is opened and said suction non-return valve is closed. 
 
     
     
       2. The oil-injection screw compressor as claimed in  claim 1 , further comprising:
 a first stop control device which computes, using the discharge side pressure detected by said pressure detecting device, a pressure rise time, the pressure rise time being the time taken by the discharge side pressure to rise from said prescribed control pressure to said prescribed upper limit, and a pressure fall time, the pressure fall time being the time taken for the discharge side pressure to fall from said prescribed upper limit to said prescribed control pressure, during the operation of said motor at the minimum number of revolutions, and stops said compressor body when the ratio between these pressure rise time and pressure fall time has reached a prescribed set ratio. 
 
     
     
       3. The oil-injection screw compressor as claimed in  claim 2 , further comprising:
 a restart control device which, while said compressor body is at a stop, restarts said compressor body when the discharge side pressure detected by said pressure detecting device has reached a prescribed restart pressure. 
 
     
     
       4. The oil-injection screw compressor as claimed in  claim 1 , further comprising:
 a second stop control device which computes, using the discharge side pressure detected by said pressure detecting device, a pressure rise time, the pressure rise time being the time taken by the discharge side pressure to rise from said prescribed control pressure to said prescribed upper limit, and a pressure fall time, the pressure fall time being the time taken for the discharge side pressure to fall from said prescribed upper limit to said prescribed control pressure, during the operation of said motor at the minimum number of revolutions, and stops said compressor body when this pressure rise time or pressure fall time has reached a prescribed set time length. 
 
     
     
       5. The oil-injection screw compressor as claimed in  claim 4 , further comprising:
 a restart control device which, while said compressor body is at a stop, restarts said compressor body when the discharge side pressure detected by said pressure detecting device has reached a prescribed restart pressure. 
 
     
     
       6. The oil-injection screw compressor as claimed in  claim 1 , wherein said blow-off valve is an electromagnetic valve. 
     
     
       7. The oil-injection screw compressor as claimed in  claim 1 , further comprising a controller configured to control operation of said motor and opening and closing of said blow-off valve. 
     
     
       8. An oil-injection screw compressor in which oil is mixed into compressed gas, the oil-injection screw compressor comprising:
 a compressor body for compressing sucked air; 
 a motor for driving said compressor body; 
 an inverter for variably controlling the number of revolutions of said motor; 
 a suction non-return valve which, disposed on a suction side of said compressor body, is configured to be opened when said compressor body is operating under load and under no load, and closed when it is at a stop; 
 an oil separator for separating lubricating oil contained in the compressed gas generated by said compressor body; 
 a compressed air supply line for supplying the compressed air cleared of the lubricating oil by said oil separator; 
 blow-off piping connecting an upper stream side of said suction non-return valve and said compressed air supply line; 
 a blow-off valve provided on said blow-off piping and capable of releasing a quantity of air beyond the discharge capacity of said compressor body when operating said motor at a minimum number of revolutions; and 
 a pressure detecting device for detecting a discharge side pressure of said compressor body 
 wherein it is controlled that, at the time of operation of said motor at the minimum number of revolutions, said compressor is run under no load to open said blow-off valve in a condition of the operation of said motor at the minimum number of revolutions when the discharge side pressure of said compressor body detected by said pressure detecting device has risen to a prescribed upper limit, and said compressor is run under load by closing said blow-off valve when it has fallen to a prescribed control pressure during the no-load running, and 
 wherein it is controlled that, when said motor is stopped after the operation of said motor at the minimum number of revolutions, said blow-off valve is opened and said suction non-return valve is closed. 
 
     
     
       9. The oil-injection screw compressor as claimed in  claim 8 , further comprising:
 a first stop control device which computes, using the discharge side pressure detected by said pressure detecting device, a pressure rise time, the pressure rise time being the time taken by the discharge side pressure to rise from said prescribed control pressure to said prescribed upper limit, and a pressure fall time, the pressure fall time being the time taken for the discharge side pressure to fall from said prescribed upper limit to said prescribed control pressure, during the operation of said motor at the minimum number of revolutions, and stops said compressor body when the ratio between these pressure rise time and pressure fall time has reached a prescribed set ratio. 
 
     
     
       10. The oil-injection screw compressor as claimed in  claim 9 , further comprising:
 a restart control device which, while said compressor body is at a stop, restarts said compressor body when the discharge side pressure detected by said pressure detecting device has reached a prescribed restart pressure. 
 
     
     
       11. The oil-injection screw compressor as claimed in  claim 8 , further comprising:
 a second stop control device which computes, using the discharge side pressure detected by said pressure detecting device, a pressure rise time, the pressure rise time being the time taken by the discharge side pressure to rise from said prescribed control pressure to said prescribed upper limit, and a pressure fall time, the pressure fall time being the time taken for the discharge side pressure to fall from said prescribed upper limit to said prescribed control pressure, during the operation of said motor at the minimum number of revolutions, and stops said compressor body when this pressure rise time or pressure fall time has reached a prescribed set time length. 
 
     
     
       12. The oil-injection screw compressor as claimed in  claim 11 , further comprising:
 a restart control device which, while said compressor body is at a stop, restarts said compressor body when the discharge side pressure detected by said pressure detecting device has reached a prescribed restart pressure. 
 
     
     
       13. The oil-injection screw compressor as claimed in  claim 8 , wherein said blow-off valve is an electromagnetic valve. 
     
     
       14. The oil-injection screw compressor as claimed in  claim 8 , further comprising a controller configured to control operation of said motor and opening and closing of said blow-off valve.